Hello!
OpenLP has some bugs (we're investigating them) concerning the use of HiDPI monitors (monitors which screen scaling is greater than 100%/96DPI). Is your second screen using HiDPI/scaling? (this can be checked on Window's Display Settings/resolution) (https://support.microsoft.com/en-us/windows/view-display-settings-in-windows-37f0e05e-98a9-474c-317a-e85422daa8bb; if it's something greater than 100% for one of your sceens, it means you're using HiDPI).
If this is the case, we suggest to disable the scaling on the second screen to see if the problem disappears.